The photographic sensitivity of a silver halide emulsion increases with the increase of the size of the silver halide grains. There is a known method of obtaining silver halide grains having increased grain size by forming the silver halide grains in the presence of a reagent called a silver halide solvent. Ammonia is perhaps the oldest known silver halide solvent. However, ammonia is only capable of increasing the grain size of silver halide within a high pH range. Therefore, the use of ammonia inevitably causes an increase in the amount of fog. Furthermore, ammonia does not have a tendency to form regular silver halide grains (i.e., the silver halide grains containing no twin grains, consisting of silver halide grains having cubic, octahedral, or tetradecahedral crystal form, and having a narrow grain size distribution) at a low pBr range (high pAg). Accordingly, the use of ammonia is not convenient because in order to obtain regular silver halide grains (group), the pAg must be strictly controlled in the low range.
Various silver halide solvents other than ammonia have recently been proposed. For example, organic thioethers are described in Japanese Patent Publication No. 11386/72. However, since the compounds are liable to fog silver halide photographic emulsions and require relatively large number of purification for preparing the compounds, the industrial use of such compounds is not practical.
Japanese Patent Application (OPI) Nos. 144319/78 (which corresponds to British Pat. No. 1,586,412) and 82408/78 (the form "OPI" as used herein refers to a "published unexamined Japanese patent application") disclose organic thione compounds such as 3,4-dimethyl-1,3-thiazolidine-2-thione and N,N,N',N'-tetramethylthiourea as silver halide solvents. However, these compounds do not desorb from silver halide grains during the washing step after forming the silver halide grains because the adsorptive power onto the silver halide grains is too strong. Accordingly, these compounds are not desirable because they suppress chemical sensitization which is latter applied to the silver halide emulsion and they obstruct the adsorption of sensitizing dyes onto the silver halide grains.
Japanese Patent Application (OPI) No. 100717/79 (which corresponds to U.S. Pat. No. 4,298,638) discloses a method of forming silver halide grains in the presence of imidazoles but the imidazole compounds disclosed therein do not tend to form regular silver halide grains (group) at a low pBr range (high pAg) as ammonia and twin silver halide grains (plate crystal) having a broad grain size distribution are formed at the low pBr range (see, Example 2).
As described above, conventional silver halide solvents do not have satisfactory properties.